Green Man Gaming

Starting at 5am Pacific on Saturday morning, GMG will offer 35% off XCOM: Enemy Unknown Borderlands 2, and Chivalry: Medieval Warfare if you use the voucher code GMG35-5BRU6-ZDRX7. From Saturday. Not yet. Games including Dishonored and Need for Speed Most Wanted will also be on sale, with up to 25% off.

Humble Bundle

Pay what you want for Splice, Eufloria, Waking Mars, Crayon Physics Deluxe, and Superbrothers: Swords and Sworcery EP, on PC, Mac, Linux and Android. Pay above the average to get Machinarium too. You'll get Steam keys for most if you pay at least $1.

Indie Gala

Pay what you want for Omegalodon, Praetorians, Majesty 2, and East India Company. Pay above the average price to also get Europa Universalis: Rome - Gold Edition, Knights of Honor, Elven Legacy, Hearts of Iron 3 Complete, King's Crusade, and Commander: Conquest of the Americas. Most can be activated on Steam.

Hearts of Iron III

Indie Royale

Pay what you want for Mutant Mudds, Pineapple Smash Crew, Spirits, BasketBelle, and Avernum: The Great Trials Trilogy. Some can be activated on Steam.

Pineapple Smash Crew

Steam

As well as being on sale all weekend, Death Rally is free for everyone to play until 1pm Pacific on Sunday. Click here to install it through Steam.
Spin the Finnish one-armed bandit to get coupons offering Steam savings. These include 50% off Alan Wake and Alan Wake's American Nightmare, 50% off Legend of Grimrock, 75% off Rochard, 75% off Trine, 75% off Trine 2, 75% off Trine Complete, and 50% off Trine 2: Goblin Menace. You can spin as many times as you please, but only ten times every five minutes.
Look, regular deals: